1. Field of the Invention
The present invention relates to a heat exchanger, and more particularly to a heat exchanger which can maintain within a certain range the temperature at an outlet port of a fluid.
2. Description of the Prior Art
In a conventional heat exchanger which includes a bypass passage, a temperature sensor provided at an outlet portion of the heat exchanger, signal transmitting means for transmitting a signal, and a changing valve provided at an inlet port of the heat exchanger for controlling the exhaust gas at the inlet port according to the signal from the signal transmitting means, the temperature at the inlet port of the heat exchanger is decreased and subsequently the temperature at the outlet port of the heat exchanger is also decreased. When the temperature at the outlet port approaches the condensing temperature, the changing valve is activated. Therefore, the exhaust gas at the inlet port of the heat exchanger is directly supplied to the outlet port of the heat exchanger through the bypass passage without heat recovery. Consequently the temperature at the outlet port of the heat exchanger is increased to a value much greater than the condensing temperature, so that effective use of the exhaust gas is not expected.